1. Field of the Invention
The present invention relates to an electrophotographic apparatus such as a copying machine, a printer or a facsimile which forms an image by using an electrophotographic system, and more particularly to an image forming apparatus which has a cleaning member for cleaning residual toner left on a surface of a photosensitive member.
2. Description of the Related Art
Generally, in an image forming apparatus such as a copying machine, a printer or a facsimile which records an image on a recording medium such as paper, an electrophotographic system is adopted as a system to record the image on the recording medium. In the electrophotographic system, a photosensitive drum as a photosensitive member, on a surface of which a photosensitive substance is coated, is used as an image bearer (or image carrier member). First, after the surface of the photosensitive drum is uniformly charged, the surface of the photosensitive drum is irradiated with a laser beam, and a potential difference is applied between an irradiated portion and a nonirradiated portion. Then, charged toner contained in a developer adheres to the surface of the photosensitive drum to form a toner image on the surface of the photosensitive drum. Subsequently, the toner image is transferred to the recording medium as an image receiving member, and an image is formed on the recording medium.
As described above, as a latent image forming system to carry out image formation by the electrophotographic system, in the case of an analog exposing system which has conventionally been used widely for the copying machine etc., noise is readily picked up. In particular, for color image formation in which image forming conditions are strict, an image forming method which includes a step of forming a dot latent image on an image bearer by switching a laser beam ON/OFF in accordance with a digital image signal has come into wide practical use. In the case of such a method, a binary recording system suffices for an image such as a character. However, the system is insufficient for reproduction of a halftone image such as a photograph which is essential. Thus, a dither method, a density pattern method, etc., have been presented as means capable of reproducing halftone images by the binary recording system.
However, since high resolution cannot be obtained by such means, as means for solving the problem, a method has been presented which can obtain a high-resolution image by modulating a pulse width (PWM) of a laser beam image signal to carry out area gradation by dots for each pixel, and forming a good halftone image without reducing a pixel density for recording. This is now a mainstream color image forming method in which image forming conditions are strict, and resolution has become higher and higher from 600 dpi to 800 dpi, and to 1200 dpi. A reduction in toner diameter is absolutely necessary to stably reproduce the high-resolution latent image and to improve image quality.
With regard to cleaning of the photosensitive member in the electrophotographic system, the surface of the photosensitive drum is repeatedly used for toner image formation many times, so that after the transfer of the toner image to the recording medium, it is necessary to sufficiently remove residual toner left on the surface of the photosensitive drum without being transferred to the recording medium. Many methods have conventionally been presented to remove residual toner. A method for scraping off the residual toner by abutting a cleaning blade which is a rubber blade made of an elastic material on the surface of the photosensitive drum in a counter direction has been put into wide practical use, because costs are low, the entire electrophotographic system can be constituted to be simple and compact, and toner removing efficiency is high. As a material of the cleaning blade, urethane rubber is generally used which is high in hardness, elasticity, wear resistance, mechanical strength, oil resistance, ozone resistance, etc.
Additionally, in recent years, polymeric toner generated by a polymerization method has been employed in place of conventional crushed toner generated by a crushing method. Because of its transfer efficiency higher than that of the crushed toner, the polymeric toner has advantages that a cleanerless system is employed, wax is easily contained for production, and no release agents are necessary when the transferred image is fixed. Moreover, sphericity of the polymeric toner is high compared with the crushed toner.
Even in the case of the crushed toner, a reduction in toner particle diameter, and shape spheroidization by considering transfer performance or the like have been carried out.
Generally, when the toner sphericity is increased, if the surface state of the photosensitive drum is unchanged, and abutting pressure of the cleaning blade is set equal to that in the case of the crushed toner, sneaking-through of the toner from the cleaning blade becomes frequent. Generally, in the image forming apparatus which uses the polymeric toner or the spheroidized crushed toner, generally, toner sneaking-through is prevented by increasing the abutting pressure of the cleaning blade or arranging a fur brush or the like as cleaning auxiliary means. As an extension of such a conventional technology, there is a method which controls driving or the like of the fur brush in accordance with a printing density to a transferring material (e.g., see Japanese Patent Application Laid-Open No. 11-212417).
However, in the aforementioned conventional example, a hardware mechanism is necessary to carry out fur brush driving control. Consequently, the apparatus becomes complex, larger in size, and costs become higher for main body.
There has been a limit on improvement of cleaning performance only by a macro image ratio over all the images.
In the aforementioned image forming method, since the latent image is formed in very small pixel units, and the latent image is developed and transferred, the transfer residual toner is left in latent pixel units on the surface of the photosensitive member. Consequently, there is a tendency that places in which transfer residual toner is generated in pixel units are numerous on a pixel center, while generation of transfer residual toner on a boundary in which pixels are adjacent to each other is limited. Especially, in order to form a high-resolution latent image for higher image quality, a film thickness of the photosensitive layer must be set small to suppress an influence of a latent image blur caused by photocarrier diffusion.
However, when the film thickness of the photosensitive layer of the photosensitive member is set small, a certain fixed value is necessary for a photosensitive member surface potential in order to obtain a developing contrast. Thus, electric field intensity on the surface of the photosensitive layer becomes higher corresponding to the thin photosensitive layer. Consequently, electrostatic attraction to toner developed in contact with the surface of the photosensitive member, especially, toner developed on the pixel center, is increased, whereby the toner tends to become transfer residual toner.
Thus, portions with much and little transfer residual toner are formed on the surface of the photosensitive member in a longitudinal direction of the cleaning blade (its orthogonal direction as well). On the portion with little transfer residual toner, sliding performance between the surface of the photosensitive member and the cleaning blade is reduced to cause partial microvibration of the cleaning blade, and the toner tends to sneak through the cleaning blade.
A particle diameter of the used toner is reduced in order to achieve high image quality. As the toner particle diameter becomes smaller, a specific surface area between the toner and the surface of the photosensitive drum becomes larger. Thus, an adhesive force of toner to the surface of the photosensitive drum per unit mass is increased to deteriorate cleaning performance of the surface of the photosensitive drum. Additionally, as the toner particle diameter becomes smaller, toner flowability is deteriorated, and a great amount of additives is necessary. Such a great amount of additives causes problems of wearing or chipping of the cleaning blade, and local line flaws on the surface of the photosensitive drum.
Furthermore, in addition to the reduction in toner particle diameter, recently, there has been an increase in cases of using the polymeric toner generated by spheroidization or polymerization. In the case of using the polymeric toner, as compared with the use of the crushed toner, toner sphericity is high, and toner sneaking-through is frequent. Thus, a linear load of the cleaning blade must be increased. Consequently, there are problems of nonuniformity of a frictional force generated between the photosensitive drum and the cleaning blade in the longitudinal direction, which is influenced by the aforementioned nonuniformity of the transfer residual toner or the like; frequent occurrence of vibration and clashing of the cleaning blade, cleaning failures, cleaning blade reversal, etc. owing to a torque increase of the photosensitive drum which accompanies an increase of a blade pressing force; and a shortened life of the photosensitive drum because of hard wearing of the photosensitive drum.